Electrical filter



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2 u? te A 'v hg!" X W. M. ROBINSON ELECTRICAL FILTER Filed Sept. 18,1942 JJJFld Aug'. 2s, 1945.

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Patented Aug. 28, 1945 mi." ll

ELECTRICAL FILTER William M. Robinson, Plainfield, N. J., assignor toCornell-Dubilier Electric Corporation, South Plainfield, N. J., acorporation of Delaware Application September 18, 1942, Serial No.458,892

4 Claims.

The present invention relates to electrical lters, in particular toanti-interference or noise lters of the type used in radio and othercommunication circuits for reducing or eliminating noise and otherundesirable interference.

In the case of communication circuits such as radio receivers designedfor high frequencies such as in the short and ultra-short wave `bandsand beyond and operated in the neighborhood of strong noise sources suchas on airplanes, automobiles, trucks, etc., diculties and very seriousproblems arise in practice. In the latter cases, expensive andcumbersome means were required in the past to properly shield all thedevices capable of generating or radiating interfering or noisefrequencies.

Many of the aforementioned defects and difllculties encountered withnoise filters of known type, especially when used in high and ultrahighfrequency circuits may be traced to the presence of an appreciableinherent series inductance caused by the leads and connections from thecapacitor section to its terminals as well as to poor contacts, whichdefects could not be reduced below a certain minimum where large andrugged condensers were necessary such as in automobiles, trucks andairplanes.

On the other hand, it is advantageous in many cases to provide a. smallinductance in series with the capacitor to obtain a lter which is mosteffective over a desired range of high frequencies to suit existingspecial conditions and requirements; in other words, it may be desirableto cause the inductance and capacity to resonate at series resonance soas to provide a minimum impedance for a desired frequency or frequencyband, such filters being known as tuned noise filters.

In the case of tuned noise llters, the series inductance may be either aseparate unit or structurally embodied in the capacitor :by theprovision of a few turns of insulated metal strip or wire connectedbetween one end of the capacitor section and the adjacent terminal(casing or insulated terminal).

According to a known arrangement of tuned filters, it hasvbeen proposedto utilize the inherent series inductance due to the connecting leadsfrom the section to the terminals in capacitors of heretofore knownconstruction to obtain series resonance at a desired frequency or over alimited frequency band. Since, however, the series inductance isdifferent in different types of capacitors and may even vary incapacitors of the same type, it is necessary to choose a capacity ofproper value in order to obtain series resonance at a desired frequency.Since, in turn, the capacity determines the effectiveness of the filteraction and, in particular, the range or band of the frequencies beingsuppressed or by-passed, at best, only a compromise solution may beobtained by using -this method which provides little flexibility inadapting it and complying with the requirements of each particular casemet in practice.

Accordingly, an object of the invention is the provision of a tunednoise or anti-interference lter wherein the inherent series inductanceof the capacitor element of the llter has been reduced to a practicallynegligible value even for the highest frequencies, in the ultra-highfrequency range and beyond, in such a manner that by the addition of aneasily Variable inductive element structurally combined with thecapacitor element, the filter may be caused to resonate effectively at adesired frequency or limited frequency range to suit existing conditionsand requirements.

Thus using a filter of the type according to the invention wherein theinherent series inductance of the capacitor has been substantiallyeliminated or reduced to a negligible value, series resonance for adesired frequency may be obtained by choosing the proper inductance inthe form of additional turns of metal wire or strip embodied in thecapacitor structure, said inductance having, furthermore, such arelation to the capacitance as to obtain a desired effective frequencyfor the filter.

In the majority of cases, the iilters are used in connection withgenerators supplying heavy currents. If the latter assumed magnitudes ofthe order of from 20 to 100 amperes or more, the conductors andterminals present both an electrical and mechanical problem. To overcomesome of the difficulties encountered, large and cumbersome devices wererequired, `which in turn increased both costs and weight. The latter isespecially serious in case of airplanes and other applications Where lowweight and bulk are of importance. When filters of the above type wereused in such installations, they had to be of substantial size and Ibulkto withstand rough handling during installation and replacement as wellas substantial vibrations and mechanical shocks during operation. Again,for this reason, the filters assumed substantial proportions as thecapacitor and additional units usually mounted in a thin metal casinghad to be provided with heavy terminal structures. This in turn made itnecessary to use a heavy container to avoid damage to the apparatus whenhandled by the ordinary workmen. For all these reasons, heavy and bulkyarrangements had to be used to hold the terminals in place and preventmovements and loosening and other obvious drawbacks and defects wellknown to those skilled in the art.

Accordingly, it is a further object of the invention to provide a tunedelectrical filter especially, though not limitatively, suitable as anoise filter or anti-interference device which is both rugged andcompact and has a minimum of size and bulk to enable its use in allcases where rough handling and exposure to vibration and heavymechanical shocks are to be expected, such as in automobiles, trucks,airplanes, etc.

With the aforementioned and other objects in view as will appearhereinafter, the invention involves basically the provision of asubstantially heavy body member or support advantageously taking theform of a metal rod or stud, which simultaneously serves as a conductorof heavy electric current ilowing through the line to which the lter isto be connected; said member also serving as a mounting support for thecapacitor section which is advantageously of the non-inductive type,spirally wound around said stud and mounted in a thin metal casingsnugly fitting the section and insulatingly supported by said stud. Inthis manner, the current carrying stud which may have a suitably largediameter and forms a part of the electrical circuit to which the lter isconnected, passes directly through the center of the lter casing so asto present a minimum of electric impedance to the current owing in saidcircuit. One side of the capacitor section is electrically connected tosaid stud, while the other side is connected to said casing, theconnections being eifected by direct soldering or clamping to avoid anyconnecting wires and resultant series inductance as in the case of theprior art filters heretofore known and used. If the casing is grounded,there is obtained in this manner a lter capacitor whose seriesinductance is reduced to a minimum, whereby the inclusion of one or morecoils of wire or the like will result in any desired limited frequencyresponse in the manner pointed out, while obtaining at the same time acompact unit of small size and weight capable of use with power orsupply lines designed to carry heavy currents and possessing extremeruggedness so as to withstand rough handling as well as heavy vibrationand shocks, both during installation and in use.

Further objects and advantages of the invention will become apparentfrom the following detailed description taken with reference to theaccompanying drawing, forming part of this specification and wherein:

Figure 1 is a vertical cross-sectional view of a capacitor constructionembodying the principles of the invention; Figure 2 is a top view on areduced scale of the capacitor shown in Figure 1 and provided with amounting lug or bracket for securing the same upon a panel or platform;Figure 3 shows a pair of capacitors with a common mounting bracketsuitable for connection to a pair of power lines; Figure 4 is a sideview of Figure 3; Figure 5 is a top View, partly in crosssection, of acapacitor according to the invention provided with means for connectingthe same to a shielded cable; and Figure 6 is a theoretical diagramexplanatory of the function of the invention.

Like reference characters refer to like parts in the different views ofthe drawing.

Referring more particularly to Figure l, the capacitor structure shownand specially suited as a filter capacitor comprises a central supportor stud I0, which may serve as a conductor for the current flowingthrough the line or circuit to which the capacitor is to be connectedand which may be designed with a sumciently large crosssection to offerlow impedance and enable the use of the filter in connection with poweror supply circuits carrying heavy electric currents. An insulating tubeII of cardboard or the like and forced over or otherwise secured to thecentral portion of the stud I0 serves as a core or mandrel for a rolledtype capacitor section I2, spirally wound thereon in a manner wellunderstood by thoseskilled in the art. The capacitor section maycomprise in a known manner a pair of metal strips, such as strips oftinfoil or aluminum separated by one or more layers of paper orequivalent insulating material, with the longitudinal edge of one stripprojecting from one side and with the longitudinal edge of the otherstrip projecting from the other side of the completed roll. In theexempliflcation shown, the projecting upper foil edges are united by alayer of solder or in any other suitable manner as indicated at I3 toform one terminal of a so-called non-inductively wound capacitor in amanner well known in the art. Similarly, the lower projecting edges 0fthe foil strip of opposite polarity are united as shown at I4 to providea cooperating terminal for the capacitor.

The projecting foil edges or terminal I3 are electrically connected tostud II) by the provision of a metal washer or flange I5, secured to orintegral with the stud and in electrical contact with the foil edges I3by mechanical pressure or preferably by soldering in such a manner as toform a rigid and compact unit, comprising the stud I0, the section I2and the washer I5. The capacitor section I2 is surrounded by a thincylindrical metal casing I6, snugly fitting over the roll and having anupper inturned annular flange I1 interposed between a pair of resilientwashers I8 and 20 of cork or the like overlying the washer I5. Washer 2Uis in turn followed by a further relatively rigid insulating washer 2Isuch as of'Bakelite or the like. A further resilient washer 22 and metalwasher` 23 are placed upon washer 2| and the entire assembly pressedinto firm engagement with the flange or washer I5 by means of a clampingnut 24 engaging the upper threaded end of stud I0 and secured to orlocked against the stud by a soldering joint as shown at 25 or in anyother suitable manner.

This construction has the advantage of relieving the section I2 so as toallow of sufficient clamping pressure to be applied to washers I8, 20-23for effecting a hermetic seal for the casing I6, I1 to prevent thepenetration of moisture or other objectionable agents into the interiorof the capacitor.

The lower projecting foil edges I4 of the section I2 forming theterminal of opposite polarity of the capacitor are electricallyconnected to the casing IB through a small inductance coil 5I) by theprovision of a metal cap having a cylindrical portion 25 tting over andsoldered or otherwise connected to the lower portion of casing I6 andhaving an inturned flange 26 soldered to or otherwise electricallyconnected to one end of coil 50 whose other end is in contact with thefoil edges I4. In order to completely seal the lower side of gamer thecapacitor against moisture penetration, there is further provided aresilient washer 21 engaging the ange 26, said washer being in turnfollowed by a relatively rigid insulating washer 28, a further resilientwasher 30 and a metal washer 3I. The superposed washers are compressedby means of a clamping nut 32 engaging the lower threaded end of stud I0and locked in the compressed position by a solder joint or the likesimilar to joint 25 for the nut 24 in such a manner as to hermeticallyseal the complete capacitor section.

There is thus provided by the construction afore-described a highlyrugged and compact as well as completely sealed capacitor mounted in athin metal casing and provided with heavy metal terminals and which iscapable of withstanding rough handling as well as mechanical vibrationand shocks and is especially adapted for use in electrical circuitscarrying substantial currents.

The inductance element 50 advantageously takes the form of a metal stripwound edgewise into a spiral having, in the example shown, three turnsseparated by insulating spacers in the form of rings 5I and 52consisting of cardboard or the like and being suitably slit for passingthe strip 50. The innermost turn of the spiral is directly soldered tothe projecting foil edges I4 of the capacitor section I2 and the outerspiral turn is soldered to the flange 26 of cap 25, the remainingelements and connections being substantially similar to the upper endconstruction and well understood from the above. This leaves a singleeffective spiral turn to provide a desired inductance andsegries'resonangevfrequency for the capacitor. If requiredfthe number of spiralturns may be increased or the diameter of the spiral varied withinlimits to provide a desired series inductance.

In order to connect the filter capacitor in an electrical circuit, thereare provided a pair of further clamping units 33 and 34 engaging thefree threaded ends of the stud I0. If the stud is connected in serieswith the circuit to be protected and the casing I6 grounded, thecapacitor will act as a filter for undesired interfering such as noisefrequencies in the manner described hereinabove.

The capacitor may be Vacuum impregnated with oil, wax, or an equivalentsubstance in the usual manner for which purpose there is provided alling hole 35 in the outer casing I6 which, after completion of theimpregnation, may be closed by lling it with solder thereby eliminatingany moisture penetration which may affect the electrical characteristicsof the capacitor. Preferably, a single layer wrapper 36 of paper or thelike is placed around the section I2, in which case the adjoining edgesof said wrapper are preferably in line with the hole 35 to facilitatethe impregnating process.

Another feature of the capacitor construction described consists in thedirect connection of the foil edges or terminals of the section to thestud I0 on the one hand and the casing I6 on the other hand in such amanner as to provide the shortest possible section-to-terminal currentpath and resulting in a minimum or practically complete elimination ofthe inherent series inductance of the capacitor. In the connection ofthe section to the stud and housing, a large crosssection of theprojected foils or edge surface insures a direct contact tosubstantially all the foils, resulting in substantial symmetricalcurrent paths of uniform impedance to the various points the connectingplug of a shielded cable.

of the capacitor section and in turn reducing the series inductance.

If it is desired to mount the capacitor filter upon a panel or platform,suitable mounting means may be provided, such in the form of a mountinglug or bracket 31 soldered or otherwise secured to the casing I6, asshown in Figure 2. In the latter, the bracket 31 is shown to be groundedat G and the stud connected in series with line L to be protected. Ifboth lines of a power or supply circuit are to be protected in themanner described above, two capacitor units are advantageously providedwith a single grounded mounting bracket as shown in Figure 3 and 4,wherein one line L1 is passing through stud of one unit and the otherline In passes through the stud of the other unit, both units beingsubstantially identical.

In the arrangement according to Figures 2 to 4, the mounting lug 31 or38 is advantageously kept as short as possible to prevent anyappreciable conducting or current path from the capacitor to ground andconsequent increase of the series inductance. Alternatively, means maybe provided to cause the current to directly enter the panel or platformupon which the unit is mounted. An arrangement of this type is shown inFigure 4 wherein the common bridging member 38 of the brackets 38 ofboth capacitor units .is slightly curved upwardly as indicated in dottedlines and possesses sufficient resiliency so that upon screwing orclamping it against the platform such as by means of screw 40, points ofincreased pressure will be established directly underneath the units 38as indicated at a: in the drawing. Accordingly therefore, the electriccurrent will directly pass from the casing of the capacitors to groundthrough the shortest possible path, thereby maintaining the seriesinductance at a minimum.

Another mode of construction, especially useful where complete shieldingof the line is desired, is shown in Figure 5. In the latter, provisionis made to connect the capacitor filter to For this purpose the unit ismounted in a cylindrical metal tube 4I, rigidly secured such as byavpress-t, soldering or the like to the outside of the casing I6. Tube4I extends sufciently in both directions and has its ends provided withouter threaded portions 42 adapted to cooperate with a threaded lockingring 45, slidably mounted upon the plug 43 of a flexible cable or anyother electrical conductor 44. In this manner the plug 43 may be tightlyscrewed against the tube 4I to electrically connect the casing I6 withthe cable shield which is usually at ground potential. In order toestablish electrical contact between stud I0 and the inner cableconductor, the plug 43 is provided with suitable flexible connectors 46in sulatingly mounted inside the plug and adapted to engage theprojecting end of the stud when the plug is applied to and tightenedagainst the capacitor unit.

Figure 6 represents a graph illustrating the effects of the seriesinductance of a lter capacitor as an anti-interference device. Thereactance in ohms of a capacitor of .0l mfd. is plotted as a function ofthe frequency. Curve a corresponds to a series inductance of about 6millimicrohenries, curve b corresponds to a series inductance of about 1millimicrohenry, while curve c represents the case where the seriesinductance is zero. As is seen in the latter case, the capacitorprovides a path of low impedance up to frequencies in the highest rangesat present being used and beyond. If an appreciable series inductanceexists, series resonance will occur at de ijnite frequencies (at and 50megacycles, `respectively, in the example shown).

From the above it is seen that the use of a substantiallyinductance-free capacitor according to the invention makes it possibleto properly choose both the capacity and series inductance so as toresult in most cases, especially within the higher frequency ranges, inan appreciable reduction of the capacity or size of the lter. Thus, forinstance, where according to the previous cumbersome methods a capacityof .1 mid. was required, the same elect was found obtainable when usinga illter according tothe invention having a capacity of .01 mfd., i. e.116 of the previous value.

It will be evident from the foregoing that the invention is not limitedto the specific construction and details described and shown herein forillustration, but that the basic principles disclosed will besusceptible, such as by the substitution of equivalent elements, ofnumerous variations and modifications coming within the broader scopeand spirit of the invention as dened in the appended claims. Thespecification and drawing are accordingly to be regarded in anillustrative rather than a limiting sense.

I claim:

l. An electrical capacitor construction comprising a relatively heavymetal stud, a noninductively wound capacitor section comprisinginter-leaved metallic foil and insulating strips and beingconcentrically mounted upon the central portion of said stud, a metalwasher soldered to the projecting foil edges at one side of said sectionand electrically and mechanically connected to said stud, a relativelythin cylindrical metal casing snugly fitting the outside of said sectionand having an inturned annular flange at one end thereof, resilientinsulating Washers encircling said stud at opposite sides of said ange,means including a nut engaging a threaded portion of said stud forcompressing said resilient Washers against said metal Washer to providean insulating hermetic seal between said stud and casing, a metal caphaving a cylindrical portion hermetically secured to the other end ofsaid casing and an 'inturned annular llange, a. metal strip woundedgewise into a spiral interposed between said section and said lastmentioned flange, insulating spacers inserted be- A tween the turns ofsaid spiral, the innermost turn of said spiral being in electric contactconnection with the projecting foil edges at the opposite side of saidsection and the outer turn of said spiral being in electric contactconnection with said last mentioned flange, further resilient Washermeans encircling said stud and engaging said last mentioned flange, andmeans for compressing said last Iwasher means into hermetic engagementwith said flange and said stud.

2*. An electrical fipacitor construction comprising a relatively heavymetal stud, a non-inductlvely wound capacitor section comprisinginter-leaved metallic foil and insulating strips and beingconcentrically mounted upon the central portion of said stud, a metalwasher soldered to the projecting foil edges at one side of said sectionand electrically and mechanically connected to said stud, a relativelythin cylindrical metal casing snugly fitting the outside of said sectionand having an inturned annular ange at one end thereof, resilientinsulating washers encircling said stud at opposite sides of said ange,means including a nut engaging a threaded portion of said stud forcompressing said resilient Washers against said metal Washer to providean insulating hermetic seal between said stud and casing, a metal caphaving a cylindrical portion hermetically secured to the other end ofsaid casing and an inturned annular flange, a metal strip wound edgewiseinto a spiral interposed between said section and said last mentionedange, insulating spacers inserted between the turns of said spiral, theinnermost turn of said spiral being soldered lto the projecting foiledges at the opposite side of said section and the outermost turn ofsaid spiral being soldered to said last mentioned ilange, and furtherresilient washer means encircling said stud and engaging said lastymentioned flange, and means for compressing said last washer means intohermetic engagement with said last mentioned flange and said stud.

3. An electrical capacitor structure comprising a cylindrical metal corehaving an annular flange at an intermediate point thereof, anon-inductive wound capacitor section mounted upon said core and havingthe protruding foil ends of one polarity directly soldered onto onesidel of said flange, a relatively thin metal tube enveloping andprojecting beyond the opposite ends of said section, means to eiect aliquid-tight seal between said core and the projecting open end of saidtube adjacent to said ange, a cylindrical cap secured to the oppositeopen end of said tube and having a central perforation spaced from andpassing said core, a metal strip wound edgewise into a spiral andinterposed between the projecting foil ends of opposite polarity of saidsection and the inside of said ca p, insulating spacers inserted betweenthe turns of said spiral, and means to provide a hermetic seal betweensaid cap and said core.

4. An electrical capacitor structure comprising a non-inductive woundcapacitor section, a cylindrical metal casing having an end wallenclosing said section, and a metal strip wound edgewise into a spiralhaving one of its end turns in electrical connection with the protrudingfoil ends of one polarity of said section and having its other end turnin electrical connection with said end wall, insulating spacers insertedbetween the turns of said spiral, Iwhereby to connect said section tosaid casing through said spiral forming an induction coil, and insulatedterminal means connected to the foil ends of opposite polarity of saidsection.

WILLIAM M. ROBINSON.

